Insert Greek letter lambda.
More robust lambda and keybinding to C-c \

Since C-\ is a standard Emacs binding, and people know how to change
it anyway.  I've also put the command in the menu for Geiser mode, for
discoverability.  A pleasant surprise: greek lambdas are understood by
both Racket and Guile.
Thanks to Ray
NEWS and version housekeeping
Little cleanups, and C-u variant for the lambda thing